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  2. INTRODUCTION The horse belongs to order Perissodactyla and family Equidae. The evolution history of horses is well known because: In North America the entire evolutionary sequence of the horse is recorded in the form of fossils within some undisrupted and uninterrupted series of sedimentary deposit ranging in age from early Eocene to Recent. Because of grazing habit, their adaptation to life on savannas and plains and their tendency to live in large herds, fossils of horse are preserved and found in large number. The evolution of horses shows orthogenesis.
  3. Adaptation of modern horses: • The horse develops elongated slender limbs with unguligrade posture. • The horse has undergone extreme evolution resulting in suppression of all the digits except the middle one(III digits) so that the horse becomes an one-toed animal • To make the quick movement most effective, the III digits of the limbs become greatly elongated and the metatarsal/metacarpal of the leg forms the powerful cannon bone. • For rapid running horses have adapted a set of spring ligaments lying on the posterior surface of foot, connecting the cannon bone with sesamoid bone and the latter with the phalanges of the hoof.
  4. • Horses have also developed a flexible and elongated neck .The growth of neck has been coupled with elongation of frontal portion of the head anterior to eyes, thus developing an elongated muzzle. • Such a result of elongation of jaw is the formation of a gap between anterior and posterior set of teeth which is called diastema. • The incisors of horse become chisel shaped with very sharp edges. Canines are of no use and hence highly suppressed . • Transformation of all premolars into molars and developing high crown hypsodont teeth which grow continuously as they are eroded throughout the life of the horse. • The teeth of the horse becomes self sharpening, self renewing and highly efficient grinding tool.
  5. The basic ancestral characters Fossil evidence indicate that condylarths are probable progenitors of the perissodactyls. Lower Eocene Hyracotherium is most primitive horse exhibiting a close morphologic similarity with the North American paleocene condylarths, Tetraclaenodon, specially as regards its structure of limbs and cheek teeth. The cusps of Tetraclaenodon were found transformed into transverse ridges or lophids. The changes from Tetraclaenodon to Hyracotherium points to a shift in adaptation for effective browsing and quick running in response to pressure from contemporaneous aggressive predators.
  7. Progressive trends of evolution Increase of size with height increasing from 12 inches to above 72 inches Lengthening of limbs with gradual shifting of ankle and wrist away from the toe. Lengthening of neck and straightening and stiffening of back producing a streamline body.
  8. Reduction of numbers of toes in limbs making all the limbs one toed. Lengthening of portion of head in front of eyes forming a mosaic. Development of high crowned cheek teeth from original low crowned brachydont type with consequent deepening of frontal part of skull and lower jaw to accommodate those teeth. Molarisation of premolars.
  9. Widening of incisors with very sharp edges. Development of complex fold pattern on enamel on the upper surface of the crown of cheek teeth. Gradual increase of complexity and size of brain.
  10. A few more points The evolution of horse shows positive allometry and negative allometry. The history of evolution of horses is still not completely explained. Cause of extinction Equus at the end Pliestocene in North America, inspite of the change of climate in a favourable direction also remains unexplained.